This invention relates to an aqueous polymer dispersion having a low acid value and low volatile organic level utilizing a polyalkylene terephthalate, or polyalkylene naphthalate, as a raw material for producing the resin. The process for making the dispersion includes an acidolysis reaction of polyalkylene terephthalate, or polyalkylene naphthalate, to produce a low acid value resin, and subsequently performing a monomer modification of the resin followed by combining the modified resin with water in the presence of a base to provide water dispersibility. More specifically, the aqueous dispersion of this invention can be an alkyd dispersion.
It has been known to employ water-reducible alkyds as binders to replace solventborne alkyds in order to reduce volatile organic content (VOC). One way to disperse an alkyd in water is to synthesize a high acid value alkyd in a water miscible solvent followed by neutralization and addition of water. Such alkyd resins and film-forming systems based thereon, however, have the disadvantages of unsatisfactory resistance to water, poor hydrolytic stability, and require considerable amounts of volatile amines and solvents for satisfactory dispersion. Another way to disperse an alkyd in water is to emulsify the alkyd in the presence of large amounts of surface-active agents. However, surface-active agents tend to impair the moisture resistance of the resulting coatings. The aqueous coating compositions produced utilizing the dispersions of this invention exhibit good hydrolytic stability and moisture resistance.
Water-reducible alkyds have also been used in aqueous coating compositions to improve the properties of latex coatings. In many instances, high gloss and wet adhesion have been difficult to achieve with latex coatings. Current water-reducible alkyds, such as those described above, when blended with a latex, may improve wet adhesion and gloss, but negatively impact stability or moisture resistance. The aqueous coating compositions produced utilizing the dispersions of this invention as a secondary binder with latex exhibit improved gloss and adhesion properties compared to unmodified latex coatings. These paints also exhibit improved stability and moisture resistance as compared to current water-reducible alkyd modified latex paints.
It has now been found that the overall performance of water-reducible polymer dispersions, more specifically alkyd dispersions, can be substantially improved by utilizing resins derived from acidolysis reaction products of polyalkylene terephthalates and polyalkylene naphthalates. According to this invention, polymer dispersions can be produced from low acid value polymers and yet have good water dispersibility. When an aqueous coating composition is formulated using the dispersion of this invention, a final coating is formed that has good hydrolytic stability, and when blended with a latex, exhibits improved gloss and wet adhesion.